Alkanes

How would you describe a cycloalkane

• A saturated molecule as there are only single c-c bonds

• The general formula CnH2n

• A hydrocarbon as there are only carbons and hydrogen

What is meant by the term fuel

A substance that reacts with oxygen in an exothermic reaction to release energy through combustion

How is crude oil separates

• fractional distillation

• Crude oil is boiled and vaporised in a furnace

• The fractionating column has a temperature gradient, hotter at the bottom and coolest at the top

• Each fraction has a different boiling point

• As gaseous fractions rise up the column, they cool and condense at different heights in the column depending on boiling point

What are the conditions for catalytic cracking (2)

500˚C and a zeolite catalyst (aluminocilocate - silica and alumina)

What is a zeolite catalyst

A catalyst used to favour the formation of short chained (branched) alkanes for motor fuels

How does catalytic cracking proceed

Via carbocatonic intermediates (carbon atom with positive charge, 3 bonds instead of 4)

What are the conditions needed for thermal cracking

450-700˚C and 60-70atm

How does thermal cracking proceed and what is the product

Via free radical intermediates and produces a higher percentage of alkenes

Explain why combustion reactions are exothermic

• energy is released when bonds are made (exothermic)

• Energy is taken in when bonds are broken (endothermic)

• More energy is released when bonds are made than then energy taken in when bonds are broken

Why are CO2 and H2O considered to be greenhouse gases

The C=O and O-H bonds absorb infrared radiation and trap it within the earth’s atmosphere. This is transferred to heat energy within the atmosphere

How does flue gas desulphurisation work

Alkaline materials such as calcium carbonate or calcium oxide react with acidic gases (ie SO2)

Write out the equations (2) for flue gas desulphurisation

CaO(s) + SO2(g) → CaSO3(s)

CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g)

Explain why car exhausts are fitted with platinum catalytic converters

Catalytic converters converters convert toxic carbon monoxide, nitrogen oxides and unburnt hydrocarbons into less harmful products

Give the equations for the following with a catalytic converter

• nitrogen oxide and carbon monoxide

• C14H30 and nitrogen monoxide

2 NO(g) + 2 CO(g) → 2 CO2(g) + N2(g)

C14H30(g) + 43 NO(g) → 14 CO2(g) + 21½ N2(g) + 15 H2O(l)

How are the covalent bonds in alkanes separated and what is formed

They undergo homolytic bond fission to form free radicals

What is homolytic bond fission

When a covalent bond is broken so that each atom takes an electron

What is a free radical

An atom or groups of atoms which have an unpaired electron

What is the halogenation of alkanes

When a hydrogen atom is substituted by a halogen atom (Br or Cl) in the presence of UV light to form halogenoalkanes and a hydrogen halide

What is the mechanism for the halogenation of an alkane

Free radical substitution

What are the 3 stages of free radical substitution mechanism

• Initiation

• Propagation

• Termination

What is the initiation stage

The formation of free radicals from a non-free radical required UV light

Why is UV needed for the initiation

The energy required to break the bond between the bromine atoms is the same as that which is provided by the UV light (enthalpy)

What happens in the propagation stage

A free radical reacts to form a product and another free radical

Give two examples of propagation reactions between clorine and methane

CH4 + CI• → •CH3 + HCI

•CH3 + Cl2 → CH3CI + CI•

What happens in the termination stage

Two free radicals form a non-radical